.Net: Remove migrated .NET Chroma connector (#14579)

### Motivation and Context

The Chroma provider has moved to CommunityToolkit/AI. Remove the
obsolete Semantic Kernel implementation and direct users to its
maintained location.

### Description

- **Provider cleanup:** Remove the .NET Chroma implementation and its
unit tests; remove their entries from `MEVD.slnx`.
- **Migration notice:** Update the Chroma README with a link to
[CommunityToolkit/AI](https://github.com/CommunityToolkit/AI/tree/main/MEVD/src/Chroma).

### Contribution Checklist

- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>
Co-authored-by: Adam Sitnik <adam.sitnik@gmail.com>
Co-authored-by: westey <164392973+westey-m@users.noreply.github.com>

Author: 3 people

dotnet/MEVD.slnx

@@ -20,12 +20,10 @@
   <Folder Name="/src/" />
   <Folder Name="/src/VectorData/">
     <File Path="src/VectorData/Directory.Build.props" />
-    <Project Path="src/VectorData/Chroma/Chroma.csproj" />
     <Project Path="src/VectorData/Milvus/Milvus.csproj" />
   </Folder>
   <Folder Name="/test/" />
   <Folder Name="/test/VectorData/">
-    <Project Path="test/VectorData/Chroma.UnitTests/Chroma.UnitTests.csproj" />
     <Project Path="test/VectorData/Milvus.UnitTests/Milvus.UnitTests.csproj" />
   </Folder>
 </Solution>

dotnet/docs/EXPERIMENTS.md

@@ -47,7 +47,6 @@ You can use the following diagnostic IDs to ignore warnings or errors for a part
 | SKEXP0010 | OpenAI file service |
 | | | | | | | |
 | SKEXP0020 | Azure AI Search memory connector |
-| SKEXP0020 | Chroma memory connector |
 | SKEXP0020 | DuckDB memory connector |
 | SKEXP0020 | Kusto memory connector |
 | SKEXP0020 | Milvus memory connector |